The basic technique employed in forming electrodeposited foil has not changed greatly over the years. In this respect, electrodeposited copper foil is generally formed by immersing a rotating drum cathode in an electrolyte solution containing copper ions. An anode formed from one or more arcuate sections of electrically conductive material is immersed in the electrolyte solution are positioned adjacent the drum cathode to define an interelectrode gap therebetween. Copper foil is formed on the rotating drum cathode by applying a current, having a current density lower than the limiting current density of the electrolyte solution, to the anode and cathode. The electrodeposited foil is continually removed from the drum cathode as it emerges from the electrolyte solution so as to permit continuous foil production.
It is well known in the art that several parameters are important in forming deposited foil of high quality and uniform thickness. For instance, maintaining a uniform, accurate spacing between the drum cathode and anode is critical to producing foil. In this respect, if the distance between the anode and cathode varies from one area to another, the cathode current density in the area of greater distance is less which reduces the deposition of metal in that area.
In some instances, a change in the interelectrode gap spacing is a matter of design choice to produce a desired characteristic in the foil produced. For example, U.S. Pat. No. 4,692,221 to Parthasarathi discloses an apparatus having plating regions having different interelectrode gaps operable to produce dendrites on the newly produced foil. To accomplish a similar result (i.e. to effect in situ surface treatment of the metal foil), U.S. Pat. No. 4,898,647 to Luce et al. discloses an apparatus having first and second anodes and a generally uniform interelectrode spacing. The first and second anodes define first and second zones wherein the second zone has a current density greater than the first zone to produce nodules on the foil. Important to both of these devices, as well as other electroforming devices, is that the designed interelectrode spacing remain uniform and constant.
Maintaining uniform spacing between anode and cathode is easier with insoluble anodes since non-uniform dissolution of soluble anodes may occur. Lead anodes are widely used in electroforming metal foils, but while lead anodes are commonly referred to as "insoluble" anodes, they are neither truly insoluble nor permanent. In this respect, in anodic usage, lead dioxide is produced at the surface of the anode and oxygen is liberated from the lead oxide surface rather than at the lead surface. Through continued usage, the lead dioxide is generally dissolved and may flake off thereby increasing the spacing between the anode and cathode and requiring increased voltage to maintain a given current density or total current for the total immersed area.
Another problem related to lead anodes is that their disposal after their useful life is gone. The proper disposal of lead and lead by-products has become a very time-consuming and expensive procedure. To maintain a uniform interelectrode gap, it is, therefore, desirable to utilize an anode material which will not react with the electrolyte solution and preferably one which does not also create the disposal problems associated with lead anodes.
Several metals, such as titanium, stainless steel, chromium, columbium, tantalum, or an alloy thereof, are generally non-reactive with electrolyte fluid and would provide the dimensional stability desired. The materials are, however, relatively poor electrical conductors as compared to lead, and anodes designs known heretofore do not lend themselves to utilization of these materials. In this respect, anode designs known heretofore only exaggerate the relative poor conductive properties of these metals in that many anode designs are generally elongated bars having either flat or curved configuration. If such anodes were formed of the afore-mentioned metals, current distribution along the surface of such bar facing the cathode would be relatively poor as compared to lead.
The present invention overcomes the problem of maintaining an accurate, uniform interelectrode gap in an apparatus for the electrodeposition of metal by providing an anode design capable of utilizing the dimensionally stable metals which are non-reactive with electrolyte solution, and by overcoming the relatively poor electrical conductive properties of such metals, thereby providing electroforming apparatus having and maintaining an exceptionally precise and uniform electrode gap, as well as improved anode service life.